Have you rebooted the phone? Since you just bought it, you might want to factory reset it. Keep in mind if you have any data on the phone, that the reset will wipe it out the phone...... so backup accordingly.
I purchased another lg stylo 5 phone after returning the last one. Same problem. I found out that removing the sim card stops that sound. Apparently the phone is trying to dial tracfone, and taking out the sim card switches the phone to wireless only or something.